1. Field of the Invention
The present invention relates to a focus detection apparatus, a method of driving the same and a camera system, and in particular, to an AF sensor for autofocus detection used in a TTL-SIR autofocus digital camera or an analog (silver salt) camera.
2. Description of the Related Art
A TTL-SIR (Through The Lens Secondary Imaged Registration) type of autofocus (hereinafter referred to as AF) sensor in related art was disclosed in Japanese Patent Application Laid-Open No. 2005-300844 by the present applicant. FIG. 9 illustrates a linear sensor arrangement layout of an AF solid-state image sensor for a linear TTL-SIR type of AF in related art. Seven linear sensor pairs 102 are formed over the same semiconductor substrate 101 of the AF sensor corresponding to the ranging points of AF. In addition, a central linear sensor pair 103 is arranged for ranging a central cross. Thus, arranging the linear sensors at the central ranging points in a cross shape in the horizontal and the vertical direction enables cross ranging for a vertical and a horizontal line detection.
Each of the linear sensor pairs 102 and the central linear sensor pair 103 have a linear sensor for an image A (standard portion) 104 and a linear sensor for an image B (reference portion) 105 as a pair of linear sensors respectively. Light beams passing through an imaging lens (not shown) of a camera system are imaged as two objects of the linear sensor for the image A 104 and the linear sensor for the image B 105 on the AF sensor by a secondary imaging optical system (not shown). The phase difference is detected to calculate a de-focusing amount.
The above linear type of AF sensor has a drawback in that a narrow ranging area in a condition where an object is the sky low in a contrast makes it difficult to obtain de-focusing amount at one time, thereby increasing the number of AF computing processes so as to decelerate the AF speed.
FIG. 10 illustrates the layout of the AF sensor for autofocus detection for an area type of TTL-SIR AF in related art. The area type of AF sensor was disclosed in Japanese Patent Application Laid-Open Nos. H11-191867 and 2005-109370 by the present applicant. In the figure, a chip is formed in the same semiconductor substrate by the CMOS process and includes a sensor circuit block 201, analog circuit block 202 and digital circuit block 203. Pixels in area sensors 204, 205, 206 and 207 have photoelectric conversion elements such as photo diodes which are two-dimensionally arranged to form an image pickup area where objects are picked up.
The TTL-SIR type of AF sensor needs an image pickup area for the image A (standard portion) and the image B (reference portion) to detect a phase difference between two objects to be ranged. That is to say, an area sensor 1′ (205) is arranged symmetrically with respect to a perpendicular line passing through an optical center 214 (or a sensor center) as the image B opposing the image A of the area sensor 1 (204). In addition, an area sensor 2′ (207) is arranged symmetrically with respect to a horizontal line passing through an optical center 214 as the image B opposing the image A of the area sensor 2 (206). In the area sensors 1 and 1′, 56 pixel arrays×18 rows are arranged. In the area sensors 2 and 2′, 145 pixel arrays×42 columns are arranged.
The analog circuit block 202 includes an auto gain control (AGC) circuit 208 for automatically controlling the gain of a signal output circuit by the accumulation time of the AF sensor, a signal amplifier circuit 209 for amplifying and outputting the photoelectric conversion signal of the AF sensor and a power supply circuit 210 for generating a reference voltage required for driving the sensor. The digital circuit block 203 includes an SRAM 211 for holding accumulation-time data of the AF sensor and gain setting value, a multiplexer circuit 212 for selecting an analog signal and outputting it to a monitor and a timing generator and input/output (I/O) 213.
In the AF sensor for autofocus detection for the above TTL-SIR AF, one line is plurally divided to enable multiple points to be ranged. On the other hand, since each ranging area is narrow, a large de-focusing cannot be focused at one time. This requires AF ranging several times, causing a problem in that AF speed is slow. In particular, the area type of AF sensor is used for AF of multiple ranging points in a wider area, however, cost is increased due to increase in chip area.
In the linear type of AF sensor, there is a limitation in which a line dedicated for large de-focusing is arranged in a predetermined area not to enable multiple points to be ranged. This, however, has the advantage that a chip area is not directly increased. On the other hand, pixels are very densely arranged in the area type of AF sensor, so that chip area is proportionally increased by the area of dedicated lines arranged, increasing cost, which prevents a practical use.